Amplifier biased too hot?

[tom1970]

Hi everyone,
Been a while since I last posted. Hope you are all well.
I've just repaired a NAD 3130 stereo amplifier which had a blown OT and I one of the drivers. It seems to be working fine but I was bored this eve and decided to check the alignment according to the instructions on p4 in the service manual (which I have attached). The centre voltage was fine but the idling current test was reading 200-300mV on both sides as opposed to 26-30mV given in the manual. The manual says adjust RX1 and RX2 to get the right current in the collector but on the actual board this is not a pot but a regular fixed value resistor on both sides (in parallel with another (R454 and R453). Given that that I need to reduce the collector current on both the left and right sides, should I be making RX1 and RX2 higher or lower? The schematic is on pp9-10.

Sorry if the question is a bit simple - I have fixed a number of amps and built guitar valve amps but my knowledge of solid state push-pull AB amplifiers (which I believe this is?) is limited.
Thanks in advance for your help.
Cheers
Tom, UK

 

[WHONOES]

Have had a look at the supplied schematic. To adjust the output idle current, use the following procedure.
Referring to the amplifier circuit at the top of the schematic.

1) Make sure that the input to the power amplifier is shorted. The easiest way to do this in to use a phono plug containing a shorting link or a very low value resistor e.g. 10 ohms or less. Also remove any speaker that may be connected so that the output is open circuit.

2) If not already done, remove any shorting link around R461.

3) If resistor RX1 is present on the PCB, remove it.

4) In place of RX1, temporarily solder the wiper and one end of a multi turn potentiometer of about 2K ohms that has been set to maximum value.

5) Connect your DMM across R461 and set to 200mV dc.

6) Turn on your amplifier and check the voltage on your DMM. It should read no more than a few mV. If not then check what you have done.

7) If all is ok then slowly adjust the added potentiometer until you DMM reads about 20 to 25 mV.

8) Whilst monitoring your DMM, leave the amplifier switched on for ten minutes or so, so that the O/P stage warms up. The voltage on your DMM will rise a little during this period. If this rise looks excessive, if it doubles for example, then adjust you potentiometer a little to bring it down. A final value of 30mV or so would be acceptable.

9) When satisfied with the result, switch off and remove the temporary potentiometer and measure its value. Select a fixed resistor that is as close in value as possible to that of the potentiometer (series or parallel resistors may be used to get as close as possible to the required value). And solder into position at RX1.
If there is room and your potentiometer is small enough, you could glue it to the PCB instead negating the need to mess about with various resistors.

10) If the above procedure doesn't work then the value of R453 may need to be increased to 200 Ohms then have another go.

11) Replace the shorting link across R461.

12) Repeat all the above for the second channel if necessary.

 

[tom1970]

Have had a look at the supplied schematic. To adjust the output idle current, use the following procedure.
Referring to the amplifier circuit at the top of the schematic.

1) Make sure that the input to the power amplifier is shorted. The easiest way to do this in to use a phono plug containing a shorting link or a very low value resistor e.g. 10 ohms or less. Also remove any speaker that may be connected so that the output is open circuit.

2) If not already done, remove any shorting link around R461.

3) If resistor RX1 is present on the PCB, remove it.

4) In place of RX1, temporarily solder the wiper and one end of a multi turn potentiometer of about 2K ohms that has been set to maximum value.

5) Connect your DMM across R461 and set to 200mV dc.

6) Turn on your amplifier and check the voltage on your DMM. It should read no more than a few mV. If not then check what you have done.

7) If all is ok then slowly adjust the added potentiometer until you DMM reads about 20 to 25 mV.

8) Whilst monitoring your DMM, leave the amplifier switched on for ten minutes or so, so that the O/P stage warms up. The voltage on your DMM will rise a little during this period. If this rise looks excessive, if it doubles for example, then adjust you potentiometer a little to bring it down. A final value of 30mV or so would be acceptable.

9) When satisfied with the result, switch off and remove the temporary potentiometer and measure its value. Select a fixed resistor that is as close in value as possible to that of the potentiometer (series or parallel resistors may be used to get as close as possible to the required value). And solder into position at RX1.
If there is room and your potentiometer is small enough, you could glue it to the PCB instead negating the need to mess about with various resistors.

10) If the above procedure doesn't work then the value of R453 may need to be increased to 200 Ohms then have another go.

11) Replace the shorting link across R461.

12) Repeat all the above for the second channel if necessary.

Thanks so much for the very thorough reply. I followed the procedure and it is pretty much sorted. Just need to tune one side a little more. Its quite precise stuff as only a few ohms difference on RX makes a huge difference on the collector current on the OT. Interesting stuff!

 

[tom1970]

BTW some people are suggesting that it is ok to leave a trim pot in the RX position rather than have it as a temporary component. Is there any reason why a trim pot is not a good idea to keep there permanently?

 

[tom1970]

Had to use a third resistor in parallel on one side to get it good as I didn't have the correct values needed so ended up with 180+820+8200 giving around 144R which produced a collector current of just over 35mA (around 5mA higher than the other side). So pretty happy with that. And the amp feels way cooler now without any seeming loss in sound quality. The centre voltages are within limits on both sides of the output. Thanks again everyone for your help. All the best. Tom

 

[WHONOES]

Thanks so much for the very thorough reply. I followed the procedure and it is pretty much sorted. Just need to tune one side a little more. Its quite precise stuff as only a few ohms difference on RX makes a huge difference on the collector current on the OT. Interesting stuff!

Hi Tom. I'm pleased that it worked out for you and your amplifier. The procedure is fairly straight forward but, if you are not familiar with class AB output stages and how they are biased, they can be a bit of a head scratcher.

Regards
Whonoes

 

[Martaine2005]

BTW some people are suggesting that it is ok to leave a trim pot in the RX position rather than have it as a temporary component. Is there any reason why a trim pot is not a good idea to keep there permanently?

Most amps I’ve serviced have trim pots. I see absolutely no reason why you can’t keep them there.
After all, a trim pot is just a variable resistor.
Down sides:
On older amps, trim pots do fail by either tolerances or oxidation. In those scenarios, I have replaced the trim pots or put in a fixed value resistor. But a fixed value leaves you with no adjustment in the future.
So in my opinion, leave them in.

 

[WHONOES]

Most amps I’ve serviced have trim pots. I see absolutely no reason why you can’t keep them there.
After all, a trim pot is just a variable resistor.
Down sides:
On older amps, trim pots do fail by either tolerances or oxidation. In those scenarios, I have replaced the trim pots or put in a fixed value resistor. But a fixed value leaves you with no adjustment in the future.
So in my opinion, leave them in.

There is certainly an argument for leaving a Pot in circuit as it allows easy adjustment. In BJT amplifiers that use the VBE multiplier biasing arrangement, the Pot should be across the Base Emitter junction of the transistor. Then if it goes open circuit, the VBE multiplier transistor gets turned hard on which then reduces idle current in the output stage to zero. The amp will not sound very nice but at least the output stage will survive to fight another day.

 

[tom1970]

There is certainly an argument for leaving a Pot in circuit as it allows easy adjustment. In BJT amplifiers that use the VBE multiplier biasing arrangement, the Pot should be across the Base Emitter junction of the transistor. Then if it goes open circuit, the VBE multiplier transistor gets turned hard on which then reduces idle current in the output stage to zero. The amp will not sound very nice but at least the output stage will survive to fight another day.

Funny you mentioned that as it brought to mind the section on VBE multipliers in this YT vid (here) as the creator makes exactly the same point. I saw this some months ago and then completely forgot about it! And in fact the maths in this bit also answer my original question about whether RX should be higher or lower in order to reduce output current.

 

[bertus]

Hello,

An other name for the vbe multiplier is rubber diode.
The wiki has a page for it:

Rubber diode - Wikipedia

en.wikipedia.org

Bertus

 

[tom1970]

Hello,

An other name for the vbe multiplier is rubber diode.
The wiki has a page for it:

Rubber diode - Wikipedia

en.wikipedia.org

Bertus

Rubber diode is a good way to describe it!

 

[WHONOES]

I prefer the term "VBE Multiplier" because that is exactly what it is.

 

[WHONOES]

Further to my comments on using "Pots" in VBE multiplier circuits. The wipers on multiturn potentiometers can be sensitive to vibration particularly post adjustment. Therefore, after making an adjustment and whilst still monitoring current flow, tap the "Pot" with a small screwdriver or something similar. If the current changes (I would be surprised if it didn't) re-adjust and tap again. Having said all that, you need to keep a sense of proportion. If the change is very minor and is tolerable, then leave as is.

Finally. I have been designing and building class AB power amplifiers for over 50 years, both BJT and FET, and have yet to experience a VBE setting pot going open circuit.
I hope this last remark is not prescient.

 

[tom1970]

Further to my comments on using "Pots" in VBE multiplier circuits. The wipers on multiturn potentiometers can be sensitive to vibration particularly post adjustment. Therefore, after making an adjustment and whilst still monitoring current flow, tap the "Pot" with a small screwdriver or something similar. If the current changes (I would be surprised if it didn't) re-adjust and tap again. Having said all that, you need to keep a sense of proportion. If the change is very minor and is tolerable, then leave as is.

Finally. I have been designing and building class AB power amplifiers for over 50 years, both BJT and FET, and have yet to experience a VBE setting pot going open circuit.
I hope this last remark is not prescient.

Brilliant. Thanks so much. Whilst the amp is fixed, I am still struggling to understand how this VBE multiplier actually works.

 

[bertus]

Hello,

Have a look at the attached pdf.

Bertus

 

[tom1970]

Hello,

Have a look at the attached pdf.

Bertus

Thanks I will study this!!!

 

[tom1970]

Hello,

Have a look at the attached pdf.

Bertus

This is great! Thanks so much. Very clear.